A variety of compounds, especially inorganic compounds are produced through a heat treatment step. In general, heat treatment is carried out by heating a compound (an inorganic compound or a raw material thereof) to be thermally treated with the compound held by a heat-resistant heat treatment container. The heat treatment container is demanded to have not only heat resistance but also stability against the compound to be thermally treated.
One example of the abovementioned inorganic compounds produced through the heat treatment step is a lithium-containing compound. The lithium-containing compound is used, for example, for a positive-electrode active material of a lithium-ion battery. Examples of the lithium-containing compound include LiMnO2-based compounds, LiN1/3Co1/3Mn1/3O2-based compounds, LiMn2O4-based compounds, LiCoO2-based compounds and LiNiO2-based compounds.
The positive-electrode active material of the lithium-ion battery (the lithium-containing compound) is produced by firing raw material powder. In general, this heat treatment (firing) of the lithium-containing compound is performed with the raw material powder stored in a container (a saggar) which was produced by firing a material having such a heat-resistant material as alumina, mullite, cordierite and spinel as a main component thereof. The saggar is disclosed, for example, in Japanese Unexamined Patent Publication No. 2009-292704.
A saggar having cordierite as a main component thereof has a high thermal shock resistance. However, since such a saggar has a high reactivity with lithium-containing compounds, there arises a problem that mixing in of a reaction product lowers purity of a lithium-containing compound after subjected to heat treatment. Especially in a case of a positive-electrode active material of a lithium-ion battery, such mixed-in impurities not only cause a decrease in battery performance of a resultant lithium-ion battery but also have a risk of becoming sources of short circuiting.
Moreover, a saggar having alumina or spinel as a main component thereof has a low reactivity with lithium-containing compounds. However, such a saggar has a high thermal expansion coefficient and has a problem that a higher content ratio of these components increases occurrence of cracking caused by thermal shock. Therefore, it has been difficult to increase the content ratio of alumina or spinel.
Japanese Unexamined Patent Publication No. 2009-292704 describes a saggar comprising spinel, cordierite and mullite. These materials have the aforementioned problems.